The present invention relates to a perforating unit of the type commonly used for making transverse lines of perforations at predetermined intervals on a weblike material, for example a web or sheet of tissue paper for the production of toilet paper, kitchen towels or the like.
A device of this kind is described in U.S. Pat. No. 5,284,304. The lines of perforations are arranged at intervals from each other defined by the type of use to which the material perforated in this way is put. For example, when this material is used for the production of rolls of toilet paper, the distance between consecutive lines of perforations is relatively small and corresponds to the length of the single sheets of toilet paper that can be torn off along the lines of perforations when the weblike material has been converted into rolls of toilet paper. In the production of rolls of kitchen towels, the interval between the lines of perforations is greater, approximately twice the interval between the lines of perforations of toilet paper. Typically, the interval between the lines of perforations is 100-140 mm for toilet paper and 220-280 for kitchen towels. Even larger distances, e.g. 380-400 mm are used for products for industrial use. Lengths of 1800-2000 mm are used for the production of rolls for medical use, e.g. for covering examination beds and folding beds.
At the present time, the perforating units used for these purposes comprise a support equipped with a basically fixed counterblade acting in combination with a certain number of rotary blades carried by a roller revolving about its own axis. The interval between the lines of perforations can be modified within certain limits (approximately 20%) by varying the speed of rotation of the bladed roller and thereby introducing slippage between the rotary blades and the weblike material passing between them and the fixed counterblade. To maintain perpendicularity between the feed direction of the weblike material and the line of perforations when this speed variation is introduced, it becomes necessary to vary the inclination of the support of the fixed counterblade and of the revolving roller relative to the feed direction of the weblike material.
The variations to the intervals between adjacent lines of perforations that can be produced in this way are not enough to convert from the interval required for toilet paper to that required for kitchen towels. In these cases it is also necessary to modify the number of blades mounted on the revolving roller. For example, if six blades are used in making toilet paper, to switch to the making of kitchen towels three of the six blades mounted on the rotary roller must be removed. Subsequent return to making toilet paper necessarily requires replacement of all blades, as the three blades that have temporarily been producing the kitchen towels will have become more worn than the three blades that were removed when switching from the production of toilet paper to kitchen towels.
The removal of some of the blades and/or the replacement of all of the blades when switching from one type of product to the other takes up a great deal of time and therefore requires long machine down times with consequent loss of output, besides high costs caused by the impossibility of using entirely all the blades.
The object of the present invention is to provide a perforating device that overcomes this problem and makes it possible to switch quickly from one type of production to another characterized by different intervals between adjacent lines of perforations, thereby reducing unused time to a minimum. Another object of the present innovation is to provide a device that does not require the blades to be replaced until actually completely worn down.
These and other objects and advantages, which will be clear to those skilled in the art on reading the text that follows, are achieved basically with a perforating unit comprising:
a support with at least two fixed counterblades;
a first roller revolving about its own axis and carrying a first series of rotary blades acting in combination with a first one of said fixed counterblades;
a second roller revolving about its axis and carrying a second series of rotary blades acting in combination with the second one of said fixed counterblades, the blades of said first series being arranged at a different pitch from the blades of the second series of blades.
With this device, in order to switch from one production to the other, the path of the weblike material need simply be modified. In one case it will pass between the first revolving roller and the fixed support, and in the other case it will pass between the second revolving roller and the same fixed support. Depending on the path of the weblike material, one or other of the two counterblades carried by the intermediate support will act in combination with one or other of the two series of blades carried by the two revolving rollers.
The different circumferential pitch between the two series of blades can be obtained with two rotary rollers equipped with different numbers of blades, for example twice as many blades on one roller as on the other. Alternatively, the two series of blades could have the same number of blades but they could be arranged on revolving rollers of different diameters. Also possible are combinations of these two solutions, i.e. the use of revolving rollers of different diameters and carrying different numbers of blades.
In one possible embodiment, one of the two revolving rollers comprises twice as many blades as the other, for example four blades on one roller and two on the other, or three blades on one roller and six on the other.
Advantageously, the support carrying the two fixed counterblades may be connected to members that modify its angular position in order to bring one or other of the fixed counterblades alternately into operation, e.g. by a slight pivoting about an axis.
As in the prior art, so in the device according to the invention the two revolving rollers and the support may have a variable inclination that can be adjusted to obtain a further possibility of variation (within narrow limits) of the interval between consecutive lines of perforations produced on the weblike material.